Low-dose radiation: thresholds, bystander effects, and adaptive responses.

I onizing radiation fills the universe. Daily ionizing particles and rays collide with molecules in 1% of the 100 trillion cells that make up the average human. These collisions generate clusters of free radicals known as reactive oxygen species that randomly damage cellular constituents including DNA (1). Certain types of ionizing radiation are more effective at generating reactive oxygen species; one -particle is at least 10 times more damaging than one -ray. To take these differences into account, the Sievert (Sv), a unit that multiplies the absorbed dose in grays (Gy) by the relative effectiveness of the particle or ray to inflict damage, was developed. On this scale, natural background radiation is 0.01 mSv day, although there are areas on earth that have values 5-fold higher (2), and spacestation inhabitants may receive 1 mSv/ day (3). At the other end of the scale, acute exposures of 150 mSv, a range known as high-dose radiation, have measurable and often serious immediate effects on humans (4). Between background and high-dose radiation is the range of exposures known as low-dose radiation. Low-dose radiation has no immediately noticeable effects on humans; nevertheless there is great interest in its long-term biological effects, which may include cancer in exposed individuals and genetic defects in their progeny. Research into the biological effects of low-dose radiation exposure is hindered by a lack of assays sensitive enough to measure the relevant cellular alterations. More-sensitive assays are being developed, an important one being the ability to detect the cellular presence of the most serious and potentially lethal type of cellular damage, the DNA doublestrand break (DSB). This assay is based on the finding that one of the highly conserved histone proteins that package the DNA into chromatin, H2AX, becomes phosphorylated at the sites of nascent DNA DSBs (5–8). The response is highly amplified and rapid, involving the phosphorylation of hundreds to thousands of H2AX molecules within minutes on several megabase equivalents of chromatin flanking the DSB. When visualized with an antibody, the phosphorylated H2AX species, named -H2AX, appears as nuclear foci (Fig. 1). In this issue of PNAS, Rothkamm and Löbrich (9) report that the number of DSBs formed, as measured by the number of -H2AX foci formed, is linear with dose from 1 mGy to 2 Gy and is also in line with pulse-field gel electrophoresis measurements at higher doses. Greatly extending previous quantitative measurements (10), their findings demonstrate that -H2AX focus formation is several orders of magnitude more sensitive than other current methods for detecting DSBs (7). How -H2AX foci function in DNA DSB repair and rejoining is poorly understood, but the foci do serve to recruit DNA-repair proteins to DSB sites (11). -H2AX foci also form as part of normal cellular processes involving DSBs, including homologous recombination during meiosis and genetic recombination during immune system development (12–15); mice lacking H2AX are viable but deficient in these two areas as well as being sensitive to ionizing radiation. Elucidating the biological roles of -H2AX foci will bring greater understanding into the various cellular mechanisms for DNA DSB formation and repair. Rothkamm and Löbrich (9) used amounts of radiation as low as 1 mGy, a dose that generates on average one track of clustered reactive oxygen species per nucleus and thus is considered the lowest dose that can affect a whole cell culture or animal. With 1 mGy, 3% of irradiated cells sustain a DNA DSB. Compared with metabolic DSBs, radiation-induced DSBs are more heter-